A high-voltage direct current transmission system generally comprises polyphase bridge rectifiers of the controlled valve type at the transmitting end of the system and polyphase bridge inverters of the controlled valve type at its receiving end.
The occurrence of disturbances in such a system is usually brought about by a violation of the correct firing sequence of the controlled valves. Such a violation takes place when a valve is fired prematurely, e.g., breaks down in forward direction or refires immediately after extinguishment, or when a valve fails to turn on in time. Such disturbances usually lead to short-circuits in the converters through valves belonging to a common phase bringing about an increase in the rectified current, i.e., the current supplied from the rectifier to the inverter, and an increase in the time interval during which current flows through the valves. This, in turn, leads to overheating of the valves and their possible damage, as well as to load dumping. Besides, the increased rectified current makes more likely a reduction in the margin angles of the other valves of the converter bridge as well as of the valves of the other bridges. The margin angle is defined as a period, in electrical degrees, for which the instant at which the anode potential of the valve rises above its cathode potential is delayed with respect to the instant at which the current in the valve falls to zero. This reduction in the margin angles of the valves is attributable to the increase in their commutation angles caused by the growth of the rectified current. The commutation angle is defined as a period, in electrical degrees, during which the current in the valve falls to zero upon application of the firing pulse to another valve of the same bridge group, which is to be fired next in turn according to the firing sequence.
An excessive reduction in the margin angle of a valve makes it possible that the valve will not have enough time to deionize completely after being extinguished and before its anode becomes positive relative to its cathode, and hence will continue to conduct. This leads to a further violation of the operation sequence, which tends to further reduce the valve margin angles, and thus to increase the probability of further development of disturbances.
As a rule, the power system coupled at the inverter side has a considerable internal impedance. In such an event the increase in the rectified current resulting from the violation of the commutation sequence gives birth to distortions in the alternating voltage at the inverter output, which tends to further reduce the margin angles of the valves and increases the probability of further development of disturbances.
To diminish the negative results caused by faulty firing sequence of the inverter valves, a method for operating a high-voltage direct current transmission system was proposed according to which, when any one of the inverter valves fails to turn on in time, the next-in-turn valve of the same bridge group is fired before its time (cf. an article by B. S. Melik-Sarkisov "Protection of a Direct Current Transmission System" in "Izvestija Nauchno-Issledovatelskogo Instituta postojannogo toka", 1957, vol. 2, p.p. 84-95). The premature firing of the next-in-turn inverter valve insures the extinguishment of the conducting valve of the same group in due time, thereby preventing the occurrence of a short-circuit in the inverter.
The premature firing of the inverter valve, however, represents an abrupt change (namely, increase) in its firing angle, i.e., the period, in electrical degrees, for which the instant of the arrival of the firing pulse is delayed relative to the instant at which the anode potential of the valve rises above its cathode potential. Such an abrupt change in the firing angle brings about a reduction in the direct current voltage across the inverter and a considerable increase in the rectified current. As was pointed out above, this tends to reduce the margin angles of the other inverter valves and adds to the probability of development of disturbances.
Besides, this method fails to protect in any way the converter valves if the violation of the commutation sequence consists in the firing of a valve before its proper time.
Another known method for controlling a high-voltage direct current transmission system consists in maintaining the smallest of the margin angles of the inverter valves at the reference level by adjustably controlling the firing angles of the inverter valves (cf. U.S.S.R. Author's Certificate No. 466,822). In this case, however, abrupt changes in the firing angles of the inverter valves also heighten the probability of development of disturbances in the same way as hereinbefore shown. Controlling the firing angles with a time delay relative to variations in the margin angles also proves to be inexpedient, since the changes in the margin angles come too late to prevent development of disturbances.
Also known in the art is a method for controlling a high-voltage direct current transmission system having a rectifier of the controlled valve type at the transmitting end of the transmission system and an inverter of the controlled valve type at the receiving end of the transmission system, which comprises maintaining the current supplied from the rectifier to the inverter in accordance with its reference value by adjustably controlling the firing angles of the rectifier valves, and reducing the reference value of the current supplied from the rectifier to the inverter when any one of the following parameters concerning the state of the transmission system goes beyond the predetermined limits: rectified current or voltage, alternating current or voltage or their time derivatives, frequency of alternating current or voltage, or power factor (cf. U.S. Pat. No. 3,801,895). This method provides a reduction in the reference value of the current supplied from the rectifier to the inverter when a short-circuit occurs in an inverter phase. The reduction in the reference value of the rectified current ensures such an adjustment of the inverter valve firing angles, which inhibits the growth of the rectified current and hence, the reduction in the margin angles of the inverter valves caused by the increase in their commutation angles, thus making further development of disturbances less likely.
In this method, however, the reduction in the reference value of the rectified current is accomplished with a certain time delay relative to the occurrence of the short-circuit. Meanwhile, a further violation of the firing sequence of other valves may take place during said time delay thereby leading to further development of disturbances.
Besides, during said time delay a load dumping occurs which adversely affects the operating conditions of the consumers' equipment.
Furthermore, deviations in the aforementioned transmission system parameters may arise from malfunctions in the process of commutation of the rectifier valves, which produce a decline in the rectified current. In this case the further reduction of the rectified current according to the known method will prevent the system from transmitting the desired amount of power, thereby unfavorably affecting the operating conditions of the consumers' equipment.
Known in the art is a regulating device for operating a high-voltage direct current transmission system having a rectifier of the controlled valve type at the transmitting end of the transmission system and an inverter of the controlled valve type at the receiving end of the transmission system, comprising a control device for adjustably controlling the firing angles of the rectifier valves, sensing means for monitoring the current supplied from the rectifier to the inverter, signal forming means for developing a signal corresponding to the reference value of the current supplied from the rectifier to the inverter, and a comparison circuit having its output connected to the control device and its inputs connected, respectively, to the current sensing means and to the output of the signal forming means, wherein said signal forming means includes a switching circuit having its output connected to the input of the comparison circuit coupled to the signal forming means, one of the inputs of the switching circuit being supplied with a signal corresponding to the value of the current supplied from the rectifier to the inverter, which insures transmission of the prescribed amount of power from the rectifier to the inverter, and a program control device having its output connected to another input of the switching circuit for switching said switching circuit and for applying a signal to the input of the comparison circuit coupled to the signal forming means, which signal causes a reduction in the reference current value when one of the parameters concerning the state of the transmission system, i.e. rectified current or voltage, alternating current or voltage or their time derivatives, frequency of alternating current or voltage, or power factor, goes beyond the predetermined limits (cf. the above-mentioned U.S. Pat. No. 3,801,895).
This regulating device features the same disadvantages as the method.